1. Field of the Invention
The present invention relates to an objective lens that is used for recording information on and/or reproducing information from an optical recording medium by using light at a wavelength of, for example, 405 nm (tolerance±8 nm). The present invention also relates to an optical pickup apparatus using the objective lens, an optical recording/reproduction apparatus using the objective lens, and a method of correcting aberration using the objective lens.
2. Description of the Related Art
A high-capacity recording medium called a Blu-ray Disc (BD, registered trademark) has been commercialized. Information is recorded on and/or reproduced from a BD by using light at a wavelength of 405 nm (tolerance±8 nm) and an optical lens having a numerical aperture equal to or greater than 0.80. Hereinafter, the term “blue band optical recording medium” refers to a Blu-ray Disc.
An objective lens used for a blue band optical recording medium has a numerical aperture equal to or greater than 0.80, which is larger than those of optical lenses used for existing optical recording media. When an objective lens has a large numerical aperture, spherical aberration becomes sensitive to variations in the thickness of the cover layer of an optical recording medium. Therefore, not only third-order spherical aberration but also fifth-order spherical aberration are not negligible. In particular, fifth-order spherical aberration is not negligible in the case of a multilayer recording medium, because a multilayer recording medium includes an intermediate layer disposed between recording layers and the thickness of the cover layer varies in accordance with the thickness of the intermediate layer.
Various mechanisms have been proposed in order to correct spherical aberration caused by a change or a manufacturing error in the thickness of the cover layer of an optical recording medium. For example, Japanese Unexamined Patent Application Publication No. 2000-131603 proposes a method of correcting aberration by moving a collimating lens along the optical axis and thereby changing the incident magnification of light entering an objective lens. With this method, aberration is cancelled by adjusting the movement of the collimating lens appropriately so that spherical aberration caused by a change in the incident magnification of the objective lens may have a polarity opposite that of spherical aberration caused by a change in the thickness of the cover layer.
Hereinafter, the term “magnification spherical aberration” refers to spherical aberration caused by a change in the incident magnification of the objective lens. Likewise, the term “disc spherical aberration” refers to spherical aberration caused by a change in the thickness of the cover layer, and the term “temperature spherical aberration” refers to spherical aberration caused by a change in temperature. Temperature spherical aberration is generated by such factors as expansion and contraction of the lens, a change in the refractive index of the material of the lens, and a change in the wavelength of incident light.
Japanese Unexamined Patent Application Publication No. 2006-31901 also proposes a technique for forming an objective lens for a large capacity optical recording medium having multi-layered recording layers while taking third- and fifth-order spherical aberrations into account.